Chiral and other photonic crystals:
spontaneous and induced emission
This project aims at creating a microcavity for a strong coupling of polarized photons with it. For single-photon source project it is very important to obtain a high Purcell factor of the cavity (significant enhancement of spontaneous emission rate in a cavity in comparison with free space) as well as deterministic polarization. We are working on the enhancement of the single-emitter fluorescence rate in cholesteric (chiral nematic) photonic bandgap microcavities as well as in ordinary organic photonic-bandgap microcavities (collaboration with V. Menon).
For better understanding photon emission in different microcavities, we are collaborating with R.W. Boyd and S.H. Chen on the efficient and robust cholesteric laser. We plan to create miniature, low threshold, polarized light sources using electroluminescent, doped liquid-crystal polymers with chiral microcavities: microlasers and light emitting diodes for medicine, telecommunications and ultrabright displays as well as electroluminescent single photon sources for quantum communication (see Cholesteric Laser page ).
In a planar cholesteric liquid crystals (CLCs), the rod-shaped anisotropic molecules with small chiral "tails" form a periodic helical structure with pitch P0 (see left Figure below for illustration and AFM images of perspective views of two CLC oligomer flakes showing a half of a pitch).
The reflectance of normally incident, circularly polarized light, with the same handedness as the CLC structure, is nearly 100% within a band centered at λc = navP0 for sufficiently thick CLC layers. The bandwidth is approximately Δλ = λcΔn/nav, where nav is the average of the ordinary no and extraordinary ne refractive indices of the medium: nav = (no + ne)/2, and Δn = ne - no.
This periodic structure can also be viewed as a 1-D photonic crystal, with a bandgap within which propagation of light is forbidden. Selective transmission curves of CLC photonic bandgap structures prepared by our group are shown below in both Figures. For emitters located within this structure, the spontaneous emission rate is suppressed within the spectral stop band and enhanced at the band edge. Figure below, right shows colloidal CdSe quantum dot fluorescence maximum at the spectral transmittance bandedge of a prepared CLC structure.
Chiral microcavity produces the polarization selectivity. We reported the first observation of single-emitter circularly polarized fluorescence of definite handedness .
Lasing experiments in dye-doped CLC structures [2, 3] with high dopant concentration confirmed that the best condition for coupling is when the dopant fluorescence maximum is at a band edge [3, 4] of the CLC selective transmission curve. See also our papers on a CLC lasing [5, 6].
Photonic bandgap samples for spontaneous emission control
see >> link << for antibunching and definite polarization experiments
(single photon source)
For sample preparation we use two types of CLCs (see Figure below):
(1) mixtures of low-molecular-weight E7 nematic-LC blend with a chiral additive CB15;
(2) oligomeric CLC powders which have liquid crystal ordering at elevated temperatures. The cholesteric or nematic state of this material can be preserved at room temperatures by slow cooling it to the glassy state with frozen cholesteric (nematic) order.
We provide details of CLC doping and sample preparation from both monomeric (fluid-like) and oligomeric CLCs (solid, glassy state) with different photonic bandgaps in papers [1, 7-10]. We use single colloidal semiconductor quantum dots, single dye molecules and nanodiamonds with single N-V color centers as fluorescence emitters.
PbSe colloidal quantum dots can fluoresce at optical communications wavelengths. Figure below shows PbSe quantum dot fluorescence spectrum and selective transmission curves of chiral photonic bandgap CLC microcavities for telecom wavelength 1.5 μm for unpolarized light (for circularly polarized light the minimum transmission value will be smaller than 5%). Left Figure: for mixtures of different ratios of components in monomeric liquid crystals; right Figure: for mixtures of different ratios of components in glassy CLC oligomers .
For chiral photonic bandgap preparation, two major aspects are important:
(1) properly choosing the concentration (or ratio) of chiral additive in a LC mixture;
(2) providing planar alignment of the CLC (buffing, photoalignment or substrate shifting).
Planar-alignment quality of glassy nematic liquid crystal samples prepared from powder oligomer is shown in Figure below. A planar-aligned, nematic liquid-crystal host provides alignment of single dye molecules in a preferred direction. Aligned dye molecules emit well-defined linearly polarized fluorescence [8, 9].
We also obtained 2-D hexagonal ordering in oligomeric cholesteric liquid crystal films during the solvent evaporation from the substrate in a specific range of oligomer concentrations . Both near-field optical microscopy (left image in Figure below) and AFM (center and right images in Figure below) showed the existence of such ordering with a periodicity of ~ 0.5 - 0.8 μm.
Combined action of convection and water droplet condensation during the evaporative cooling are the suggested mechanisms of the observed hexagonal patterns. See also papers [12-13]. Cooling by the evaporating solvent leads ambient moisture to condense on the hydrophobic mixture. Water droplets segregate and entrap into these self-assembling, hexagonally-ordered pattern Within minutes, the system returns to equilibrium with the water evaporated from the cavities leaving the air bubbles on the film surface.
The second type of photonic bandgap structure doped with single CdSe quantum dots was prepared for us by Menon's group (Queens College of CUNY) using solution processing. The details of structure preparation are reported in . The distributed Bragg reflectors (DBRs) are fabricated by spin coating alternating quarter wavelength thick polymers' layers with different refractive indices n. Greater than 90% reflectivity is obtained using ten periods of the DBR structure. The 1-D microcavity is formed by sandwiching a defect layer doped with single quantum dots between two such DBRs. The top and bottom DBRs both comprise ten periods.
Figure above, left shows the reflectivity of the whole structure (DBRs with a defect layer between them) with the cavity mode at ~620 nm. The quality factor (Q) of such a microcavity was found to be ~40. The inset of this Figure shows the normalized reflectivity spectra of the DBR with ten periods without any defect layer. Fluorescence imaging by our group of single CdSe quantum dots in a DBR structure with a defect layer is shown in Figure above, right. This raster scan image shows blinking of single QDs (bright horizontal stripes) and a low host fluorescence background, making these structures suitable for single photon source .
- S.G. Lukishova, L. J. Bissell, V.M. Menon, N. Valappil, M.A. Hahn, C.M. Evans, B, Zimmerman, T.D. Krauss, C. R. Stroud, Jr., R.W. Boyd, "Organic photonic bandgap microcavities doped with semiconductor nanocrystals for room-temperature single photon sources on demand", J. Modern Optics, Special Issue on Single Photon, Vol. 56, No 2-3, 20 Jan.- 10 Feb., 167-174 (2009). View pdf
- I.P. Il'chishin, E.A. Tikhonov, V.G. Tishchenko and M.T. Shpak, "Generation of a tunable radiation by impurity cholesteric liquid crystals", JETP Lett., Vol. 32, 24-27 (1980).
- V.I. Kopp, B. Fan, H.K.M. Vithana, and A.Z. Genak, "Low-threshold lasing at the edge of a photonic stop band in cholesteric liquid crystals", Opt. Lett., Vol. 23, 1707-1709 (1998).
- J.P. Dowling, M. Scalora, M.J. Bloemer, and C.M. Bowden "The photonic band-edge laser: a new approach to gain enhancement,", J. Appl. Phys., Vol. 75 , 1896-1899 (1994).
- S.K.H. Wei, Shaw H. Chen, K. Dolgaleva, S.G. Lukishova, R.W. Boyd, "Robust organic lasers comprising glassy-cholesteric pentafluorene doped with a red-emitting oligofluorene", Appl. Phys. Lett., Vol. 94, 041111 (2009). View pdf
- K. Dolgaleva, S.K.H. Wei, S.G. Lukishova, S.H. Chen, K. Schwertz, and R.W. Boyd, "Enhanced laser performance of cholesteric liquid crystals doped with oligofluorene dye", J. Opt. Soc. Am. B, Vol. 25, Issue 9, pp. 1496-1504, 2008. View pdf
- S.G. Lukishova, A.W. Schmid, Ch. M. Supranowitz, N. Lippa, A. J. McNamara, R.W. Boyd, C.R. Stroud, Jr., "Dye-doped cholesteric-liquid-crystal room-temperature single photon source, J. of Modern Optics, Special Issue on Single Photon: Detectors, Applications and Measurements Methods, Vol. 51, No 9-10, pp.1535-1547, 2004. View pdf
- S.G. Lukishova, A.W. Schmid, R. Knox, P. Freivald, L. Bissell, R.W. Boyd, C.R. Stroud, Jr, K.L. Marshall, "Room-temperature source of single photons of definite polarization", J. Modern Optics, Special Issue on Single Photon: Sources, Detectors, Applications and Measurement Methods, Vol. 54, iss. 2 & 3, pp. 417-429, 2007. View pdf
- S.G. Lukishova, A.W. Schmid, R.P. Knox, P. Freivald, A. McNamara, R.W. Boyd, C.R. Stroud, Jr., K.L. Marshall, "Single-photon source for q uantum information based on single dye molecule fluorescence in liquid crystal host", Molec. Cryst. Liq. Cryst., Vol. 454, pp. 403-416, 2006. View pdf
- S.G. Lukishova, L.J. Bissell, J. Winkler, C.R. Stroud, "Resonance in quantum dot fluorescence in a photonic bandgap liquid crystal host", Optics Letters, Vol. 37 (7), 1259-1261 (2012).
- S.G. Lukishova and A.W. Schmid, "Near-field optical microscopy of defects in cholesteric oligomeric liquid crystal films", Molec. Cryst. Liq. Cryst., Vol. 454, pp. 417-423, 2006. View pdf
- M. Srinivasarao, D. Collings, A. Philips, S. Patel, "Three-dimensionally ordered array of air bubbles in a polymer film", Science, Vol. 292, 79-83 (2001).
- G. Widawski, M. Rawiso, & B. Francois, "Self-organized honeycomb morphology of star-polymer polystyrene films" Nature, Vol. 369, 387-389 (1994).
- V N. Valappil, M. Luberto, V.M. Menon, I. Zeylikovich, T.K. Gayen, J. Franco, B.B. Das, R.R. Alfano, "Solution processed microcavity structures with embedded quantum dots", Photon. Nanostruct., Vol. 5, 184-188 (2007).